ensure the list of note types is not empty

fixes an issue opening the add screen if a user manages to delete
all their note types with an add-on or a sync

anki/collection.py

@@ -872,6 +872,9 @@ and type = 0""", intTime(), self.usn())
         curs.execute("update revlog set ivl=round(ivl),lastIvl=round(lastIvl) where ivl!=round(ivl) or lastIvl!=round(lastIvl)")
         if curs.rowcount:
             problems.append("Fixed %d review history entries with v2 scheduler bug." % curs.rowcount)
+        # models
+        if self.models.ensureNotEmpty():
+            problems.append("Added missing note type.")
         # and finally, optimize
         self.optimize()
         newSize = os.stat(self.path)[stat.ST_SIZE]

anki/models.py

@@ -96,10 +96,17 @@ class ModelManager:
     def flush(self):
         "Flush the registry if any models were changed."
         if self.changed:
+            self.ensureNotEmpty()
             self.col.db.execute("update col set models = ?",
                                  json.dumps(self.models))
             self.changed = False
 
+    def ensureNotEmpty(self):
+        if not self.models:
+            from anki.stdmodels import addBasicModel
+            addBasicModel(self.col)
+            return True
+
     # Retrieving and creating models
     #############################################################